In the recent accelerating activity with respect to metal, plastics, and composite prostheses for various joints in the human body, a great deal of attention has been paid to the knee joint. As is well known, this joint must absorb substantial stress, primarily loadings directed axially of the adjacent leg portions, but also some flexing stresses. In fact, the vertical loading on the knee joint of a 200-pound man often reaches values as high as at least 800 pounds per square inch.
There have in the past been designed a number of prosthetic knee joints intended for manufacture from plastics materials, or from plastics and metal materials, wherein large surfaces have been provided as bearing surfaces for the support of these loads and to reduce the unit stress thereon and these designs have had many good features. However, as for example in the patent to Lagrange, U.S. Pat. No. 3,918,101, wherein this basic concept is developed, there is also provided a centrally split housing to define the joint cavity. In this case, the two parts of such housing are held together by screws and hence in conditions of use involving tension through the joint such screws will be subjected to tension forces. Since these forces must be resisted by the thread screws in the plastics material, this presents a point of weakness.
In other proposals, such as Goldberg U.S. Pat. No. 3,765,033, wherein the broad concept of providing wide support for the joint loading is suggested, the components are fastened together by a transversely positioned pin in an oversize bore. This not only requires quite precise original dimensioning but also inevitably provides for looseness in the joint, particularly when the user's leg is under tension and it would be unsatisfactory.
Various other disclosures recognize the problem and attempt to solve it by providing wide support surfaces as above mentioned but they all have other features involving either looseness, undesirable concentration of forces in certain use situations with resulting points of weakness or other undesirable constructional and/or operational features.
Accordingly, the objects of the invention include:
1. To provide a prosthetic knee joint which can be formed from plastics materials and/or plastics-metal materials as desired, which will provide heavy load capabilities in the normal condition of use as in walking by the user, but which will provide firm and smooth bending of the knee under all circumstances of use and permit no looseness therein even under conditions of lateral flexure or leg tension.
2. To provide a prosthesis, as above mentioned, wherein the load in all directions of major magnitudes is at least partially carried by a solid piece of material and wherein the rest of such load is carried by shear stress on the screws holding component parts together, and only very minor loadings will apply tension stress to such screws.
3. To provide a prosthesis, as above mentioned, wherein the joint will be firm and solid and wherein particularly there will be no free play present.
4. To provide a prosthesis, as aforesaid, which will be of sufficiently simple construction as to be formable in relatively large masses by molding and will be free from complex parts and consequent difficulties in molding.
Other objects and purposes will be apparent to persons acquainted with devices of this general type upon reading the following specification and inspection of the accompanying drawings.